This invention relates to a method for converting methanol and/or dimethyl ether to a hydrocarbon product, particularly a hydrocarbon product rich in isobutane and isobutene. The invention is also concerned with new catalysts, by means of which methanol and/or dimethyl ether is converted to a hydrocarbon product rich in iso-C.sub.4 compounds.
Isobutene or isobutylene and isobutane are important starting materials for the preparation of a variety of commercially important products, including maleic anhydride-isobutylene copolymer, 2,5-dimethylhexadiene, which is a key intermediate in the manufacturer of pyrethrin insecticides, butyl rubber, polybutenes, methyl methacrylate, alkylate, which is a blending component for high octane gasoline and methyl t-butyl ether, which is also a blending component for high octane gasoline.
At present, isobutylene is obtained generally from the cracking of liquid petroleum or from naturally occurring field butanes. However, the demand for isobutylene far exceeds the supply, and this demand is increasing.
The synthesis of branched chain hydrocarbons from synthesis or water gas, i.e., a mixture of carbon monoxide and hydrogen, has been investigated at the Kaiser Wilhelm Institute For Coal Research, Germany; according to a translation of a report by Pichler and Ziesecke, Bureau Of Mines Bulletin, 488, U.S. Government Printing Office, Washington, D.C. 1950, water gas has been converted under high pressure and elevated temperature to a hydrocarbon product containing isobutylene by means of a variety of catalysts, including catalysts composed principally of an oxide of thorium, aluminum, tungsten, chromium, titanium, zirconium, uranium, zinc, manganese or cerium and mixtures of thorium oxide with alumina, zinc oxide, chromium oxide, iron and copper.
While the work of Pichler et al. shows that water gas can be converted into a hydrocarbon product containing iso-C.sub.4 hydrocarbons, the Pichler et al. process has serious shortcomings which have prevented its commercial use. In particular, the conversion is carried out at very high pressure, i.e., from 30 to 1,000 atms.; the higher the pressure, the better the conversion; however, high pressure equipment is very expensive. Further, thorium oxide is also costly and the concentration of thorium oxide in the Pichler et al. catalyst is high, i.e., about 71-100 percent by weight.
More recently, crystalline aluminosilicate zeolites or molecular sieves have been disclosed and shown to be useful for catalyzing a variety of reactions. For example, according to U.S. Pat. No. 3,036,134, alcohols may be converted to ethers, by means of a crystalline aluminosilicate of the formula: EQU M.sub.2/n O:Al.sub.2 O.sub.3 :XSiO.sub.2 :YH.sub.2 O
wherein M is a metal and n its valence, X varies from about 1.35 to 3 and the average value for Y is between 4 and 8.
The dehydration of a normal alcohol to an olefin having a corresponding structure by means of such a synthetic zeolite or molecular sieve is disclosed in U.S. Pat. No. 3,529,033.
According to U.S. Pat. No. 4,079,096, a catalyst of crystalline aluminosilicate zeolite is also useful to convert methanol and/or dimethyl ether to a hydrocarbon product rich in ethylene and propylene. Reactions of polar organic compounds through the use of zeolite catalysts are disclosed in U.S. Pat. No. 3,728,408; the production of aromatic compounds from liquid hydrocarbons using a crystalline aluminosilicate catalyst is taught as U.S. Pat. Nos. 3,756,942 and 3,760,024, and the aromatization of alcohols, ethers and carbonyl-containing compounds by means of a crystalline aluminosilicate zeolite in U.S. Pat. No. 3,894,104. The use of a crystalline aluminosilicate zeolite catalyst to convert lower aliphatic alcohols, carbonyls, ethers and the like to a hydrocarbon product rich in compounds containing 5 or more carbon atoms has been disclosed in U.S. Pat. No. 3,894,103, while the conversion of compound of formula CH.sub.3 X, wherein X is a hydroxyl, alkoxy, alkylthio, amino, alkylamine dialkylamine, halo or cyano group to aromatic compounds is taught in U.S. Pat. No. 3,894,105, and the conversion of methanol and/or dimethyl ether to a hydrocarbon product rich in ethylene and propylene is taught in U.S. Pat. No. 4,062,905.
German Pat. Nos. 2,827,385 and 2,755,229 also concern the conversion of methanol and/or dimethyl ether by means of a zeolite catalyst to a hydrocarbon product rich in ethylene and propylene.
Recently, a crystalline silica composition of uniform pore diameter, which exhibits molecular sieve and hydrophobic organophilic properties, but not ion exchange properties has been disclosed by Gross et al. in U.S. Pat. No. 4,061,724, which is incorporated herein by reference. According to Gross et al., this silica composition, which is referred to as silicalite is substantially free of alumina except for incidental impurities and the crystals thereof are orthorhombic. Silicalite has also been characterized as a silica polymorph, which after calcination in air at 600.degree. C. for 1 hour, has a mean refractive index of 1.39 and a specific gravity at 25.degree. C. of 1.70. Due to its uniform pore structure, silicalite is disclosed as being useful to separate p-xylene from o-xylene and as being useful for selectively absorbing organic materials from water as a result of its hydrophobic/organophilic properties.